Resistance changing weed control approach

It’s no secret that glyphosate resistance has become a significant problem for Georgia cotton producers, and researchers are finding that managing this pest is influenced by many factors.

“In areas where we’re fortunate, and we have rainfall or can irrigate, we can be fairly successful managing glyphosate-resistant Palmer amaranth pigweed. But in other areas, it’s far more challenging,” said Stanley Culpepper, Georgia Extension agronomist, speaking at the recent Beltwide Cotton Conferences in San Antonio.

Before a management program can be developed, it’s important to know exactly what growers are doing, says Culpepper. “This is a fairly new concept, because in the past, I only had to know which weed species was in your field. Now, many more factors must be considered,” he says.

These factors include whether or not the field is irrigated — conventional-tillage, dryland — conventional-tillage, dryland — conservation-tillage, or irrigated — conservation-tillage.

“Usually, if it’s irrigated, we do very well, as long as we can convince the grower to turn on the irrigation within a day or two of applying the residual herbicide. But dryland is where we’re struggling desperately,” he says.

Everyone knows, he adds, that residual herbicides are a strong component of any program to manage glyphosate-resistant Palmer amaranth. “If you were in the central part of Georgia last year, near Tifton, and you wanted to get an activating rainfall, you had five to seven days when you would get at least one half inch of rain.

“It’s a challenge for us in the southern and central part of the state to get these residuals, especially at-plant herbicides, activated by rainfall. That is why we’re struggling so desperately in dryland production,” says Culpepper.

There is no doubt, in Georgia, that an integrated approach will be needed to manage this pest, he says. Researchers are now in the process of attempting to understand and determine which specific “tactics” could be used in an integrated approach, he says.

These tactics could include tillage, cover crops, Ignite-based programs, or various mixtures of all of these, says Culpepper.

“Is is tillage? Is it cover crops? We’re 98-percent Roundup Ready now, so should we be using more Ignite-based programs? Or will it be a mixture of all these plus additional control measures such as hand weeding?”

Culpepper, along with other Extension specialists and researchers, conducted a study looking at the impact of soil tillage activities such as deep turning, incorporating a yellow herbicide, and cultivating in a dryland, conventionally produced crop.

Two experiments were conducted in Macon County, Ga.

The first tillage option was either breaking the land or not breaking the land, he explains. “Our four herbicide options were no herbicide, a WeatherMax or glyphosate-only program, and we had two systems that we refer to as the Prowl system and the Treflan system. The Prowl system was an application of Prowl H20 plus Reflex pre-emergence, followed with a topical application of WeatherMax and Dual and a layby of conventional chemistry — Direx plus MSMA,” says Culpepper.

The Treflan system included incorporating a yellow herbicide — a lost art among some growers, he says. “We incorporated Treflan, followed with Reflex pre-emergence, and then followed with Direx and MSMA, the same as for the Prowl system. Herbicide use rates were typical for the soils in which we were producing these crops.”

The trials were small plots, says Culpepper, and they received rainfall five days after application. “Our residual at-plant herbicides in this trial are going to be far more productive than in a typical situation where it may not have rained for 10 to 15 days.”

When cotton reached the one to two-leaf stage, it became evident that breaking the land was an extremely important factor, he says.

“Even when we didn’t apply a herbicide, the difference between breaking land and not breaking the land was very evident. When you counted all replications, it was about a 60-percent reduction in the number of Palmer amaranth plants that were present on two-week cotton in the no-herbicide systems. We all know that managing this pest is a fight against the seed banks and the population we have in the field.

Looking at Palmer amaranth control at mid- to late-season prior to layby treatments, the deep tillage treatments featured the Roundup Ready-only system and the Prowl and Treflan systems.

“When we didn’t have deep tillage, it was obvious the Treflan system was pretty effective. But even at 89 percent, we’re not even at layby, and it is a potential challenge. If you look at deep tillage systems, we have 53 percent visual control with no herbicide. This is good news for me, because for several years, nothing by itself would give us 53 percent control. The Roundup Ready-only system was 67 percent and the Prowl and Treflan systems were phenomenal. For the first time in four or five years, we saw a program with acceptable control.”

At harvest, the advantage of the Treflan over the Prowl system without deep tillage is still evident, at 90 percent to 79 percent, but both programs — Prowl at 94 percent and Treflan at 98 percent — had excellent control in deep tillage.

“We used the exact same herbicide programs — we simply had fewer plants emerging where we deep turned the land. Although we may have removed the same percentage of plants, we could at least harvest our crop where we deep turned the land.”

Seed cotton yields, says Culpepper, followed very closely the same trends in the trial as Palmer amaranth control. Where the ground was not broken, the Treflan system was more productive than the Prowl system. Where deep tillage was used, the Prowl and Treflan systems were statistically similar. “And we could not harvest where we used no herbicides or where we had three applications of glyphosate.”

The second experiment, he says, consisted of larger plots but essentially the same treatments. There were two herbicide treatments, each with and without cultivation, and they were identical to the Prowl and Treflan systems used in the previous trials.

“We saw similar results,” says Culpepper. “At harvest, incorporating Treflan gave us about 10 percent better control versus putting Prowl H20 on top of the ground. This experiment also received rainfall five days after herbicide application, and that’s critical because if it was 10, 15 or 20 days after application, my guess is that Treflan would have been even more effective.”

The benefit from cultivation was seen in the Prowl system with 11-percent better control, he says. “We did not have a significant advantage in the Treflan system, but with the mentality that we often need 98 or 99 percent control for this pest, it may be enough to be a practical advantage for your growers.”

Yields, says Culpepper, followed almost identical trends. “We did have a significant advantage within each system where we cultivated. The benefit from that cultivation — even in the Treflan system — showed up in our yield comparison.

Work also was done in Alabama three to four years ago, with numbers generally ranging between 60 percent and 70 percent reduction of various weed species, including glyphosate-resistant Palmer amaranth.

“When we’re not deep turning the land, which would include the majority of our conservation-tillage, incorporating a yellow herbicide improved control by 11 percent at harvest and increased yields by 26 percent. We understand the limitations of this for growers — we’re just trying to find programs that are effective.”

Cultivation, he says, improved control in the Prowl system by 11 percent and increased yield by 6 to 10 percent in the Prowl or Treflan systems.

“In conclusion, deep turning the land, incorporating the yellow herbicide, and cultivation all can be used to improve the control of glyphosate-resistant Palmer amaranth in dryland Roundup Ready cotton production in Georgia.”